Certain photolithography parameters, including the variation of critical dimension (“CD”) printed with pitch, otherwise sometimes referred to as Optical Proximity Effect (“OPE”), in a scanner imaging system, show a behavior that is characteristic of the imaging and process conditions and is sensitive to variations in those conditions. Maintaining stable process conditions can improve the effectiveness of mask Optical Proximity Correction (“OPC”) used to offset the effects of OPE. One of the factors which affects the OPE is the spectral bandwidth of the light source. To date, passive bandwidth stabilization techniques have been effective in meeting OPE control requirements. However, future tighter OPE specifications, among other things, will require advanced bandwidth control techniques. Applicants believe that future tighter OPE specifications will require active control techniques to both improve the stability of E95 bandwidth, and also regulate E95 bandwidth to a desired setpoint (i.e., within a selected very narrow range). The recent work of Huggins et al., “Effects of laser bandwidth on OPE in a modern lithography tool.”, Optical Microlithography XVIII (2006), describes how controlling the spectral properties of the laser light, specifically E95 bandwidth, has an effect of similar magnitude to those from other control parameters, such as focus shift, dose shift and partial coherence shift. The bandwidth metric, E95, is defined as the width of the spectrum (typically in picometers) that contains 95% of the integrated spectral intensity. A second bandwidth metric that is commonly employed is the Full Width at Half-Maximum (FWHM), which, although easier to measure than E95, does not affect OPE as significantly.
State of the art on board metrology, used to accurately measure E95 bandwidth, as discussed in one or more of the above referenced co-pending U.S. patent applications, has enabled a new array of active control solutions to be deployed. Advanced spectral engineering techniques, including sophisticated control algorithms, according to aspects of an embodiment of the disclosed subject matter, are disclosed to be able to be used to stabilize and regulate the bandwidth of the lithography light source while maintaining other key performance specifications.
In dual chamber lasers, such as in a master oscillator/power amplifier (“MOPA”) configuration or master oscillator/power oscillator (“MOPO”) configuration, bandwidth, such as, E95 bandwidth, may be seen to be sensitive to the relative time delay, denoted ΔtMOPA, between the commutation of the MO and PA/PO pulse power. The MO output pulse becomes more line-narrowed over its duration, having traversed the line narrowing module more often. Consequently, as the PA chamber is fired later relative to the MO chamber, it selects a more line-narrowed portion of the MO pulse and the effective E95 bandwidth of the output of the entire laser system also decreases. FIG. 2 shows how E95 bandwidth can vary as differential firing time is adjusted, by way of example only and not limited to, on a typical MOPA configuration dual chamber laser system. It will be understood that differential firing time or dtMOPA or ΔtMOPA are all short hand expressions for the concept of the difference in the timing of the electric discharge between the pair of electrodes in the seed laser and the pair of electrodes in the amplifier laser, whether specifically the amplifier laser is a power amplifier, an amplifier configured as a power oscillator or a power ring amplifier (an oscillating amplifier with a ring path through the amplifier gain medium).
According to aspects of an embodiment of the disclosed subject matter applicants propose developments in active stabilization of bandwidth.
A variable magnification line-narrowing module is described in U.S. Pat. No. 6,393,037 which issued to Basting et al., on May 21, 2002 (“Basting”), the contents of which are hereby incorporated by reference herein. The abstract of Basting describes a tunable laser including an angular dispersive element and a beam expander including one or two rotatable prisms along with a grating in a line narrowing module to adjust the bandwidth resulting from adjusting the magnification of the beam incident on the dispersive element. The prism beam expanders, when two are used, are disclosed to be mechanically linked to so that the angle of incidence of the beam on the dispersive element is not changed when the magnification changes. This arrangement makes it very difficult, if not impossible, to control center wavelength as well as bandwidth utilizing the rotatable prisms.
Several algorithms may be utilized, including an E95 feedback algorithm, a laser power feed forward algorithm, a dither control algorithm and a bandwidth control device (“BCD”, such as a mechanism to modify the curvature of a dispersive wavelength/bandwidth selection dispersive optical element like a grating) curve trace algorithm. Applicants have proposed, by way of example, to use a measured E95 signal to determine an adjustment to the BCD position, with the aim of staying on a particular side of a BCD operating curve. This is illustrated by way of example in FIG. 1 of U.S. patent application Ser. No. 11/510,037 entitled, ACTIVE SPECTRAL CONTROL OF DUV LIGHT SOURCE (“the '037 Application”), FIGS. 1 and 2 of which are incorporated herein.
FIG. 2, also from the '037 application, illustrates an E95 bandwidth control authority using ΔtMOPA. The use of differential firing time as a fine actuator to control E95 bandwidth has a number of advantages, including, (1) the measurement of E95 and the change of ΔtMOPA both can occur on about a tens-of-pulses time scale, or shorter, such as, pulse-to-pulse, allowing for very high frequency disturbance rejection; and (2) the available range of actuation is large enough to attenuate/suppress the sources of bandwidth deviation being targeted, namely laser energy and the higher frequency effects of duty cycle variations.
Rafac, “Overcoming limitations of etalon spectrometers used for spectral metrology of DUV excimer light sources”, Optical Microlithography XVII, Bruce W. Smith, Editor, Proceedings of SPIE, Volume 5377 (2004) pp. 846-858, which is hereby incorporated by reference, discusses methods and apparatus for calculating bandwidth, such as E95 bandwidth. Other such wavemeters for performing measurements, calculating center wavelength and/or bandwidth and producing an output signal indicative thereof are well known in the art, e.g., as disclosed in U.S. Pat. No. 6,894,785 titled, “GAS DISCHARGE MOPA LASER SPECTRAL ANALYSIS MODULE” which issued on May 17, 2005, and in U.S. Pat. No. 6,539,046 titled, “WAVEMETER FOR GAS DISCHARGE LASER” which issued on Mar. 25, 2003, both of which are hereby incorporated by reference. In addition, techniques for calculating FWHM and E95 from wavemeter output data are disclosed in U.S. patent application Ser. No. 10/615,321, filed on Jul. 7, 2003, and entitled, “OPTICAL BANDWIDTH METER FOR LASER LIGHT”, and U.S. patent application Ser. No. 10/609,223, filed on Jun. 26, 2003, and titled, “METHOD AND APPARATUS FOR MEASURING BANDWIDTH OF AN OPTICAL OUTPUT OF A LASER”, both of which are hereby incorporated by reference herein.
GigaPhoton is believed to advertise a product that performs E95 control using some sort of optical actuation. Japanese Published Patent Application 2006024855, published on Jul. 9, 2004, discloses a variable magnification LNM, also with two rotatable prisms and the use of a differential discharge timing for bandwidth control. Such an arrangement makes it difficult, if not impossible, to control center wavelength and bandwidth with the prisms.